Sidebar shows ~6000 photos Unable to Upload, but Smart album shows none

If you are seeing an album in the side bar with "unable to upload", trust this album.

What kind of media are in this album? Are there videos in the album that cannot play? Photos that cannot be be viewed at large? The photos and videos in this album may need to be converted to a format, that is compatible with Photos 5. Catalina does no longer support all formats that have been supported in the previous system versions.

• I would create an own album with all photos and videos from the "unable to upload" section in the sidebar, because this album may change over time.
• Once you have collected the items in your own album, export all items from this album as described here: Check the status of your library in Photos for macOS - Apple Support

1. Select your own "Unable to Upload" album in the left sidebar (your standard album, not the empty smart album).
2. Press Command-A to select all the photos in the album, and then choose File > Export > Export Unmodified Original For (#) Photos.
3. In the dialogs that follow, choose the file name format and location for the exported photos. Click Export, then click Export Originals.
4. After the photos export, make sure the photos are all still selected in the album. Delete all selected photos with the shortcut ⌘⌫ . remove them from recently deleted as well.
5.  To import the photos back into your library, choose File > Import and find the images in the location you chose in step 5.

If photos or videos cannot be imported because of an incompatible format, try to convert them to a different format .

You may need a third party application.